Treat an All-or-Nothing Change as a Transaction
Commit the complete valid change or roll back the incomplete change.
About 22 minutes, plus any extended practice
Supplied materials
- Paper and pencil; answers may also be spoken or typed.
- All cases, labels, numbers and rules are supplied on this page. No real account, private information or purchase is needed.
Learn the idea
A transaction groups operations so the defined change can commit together or be rolled back. In our atomic transfer model, debiting A without crediting B is unacceptable. Transaction boundaries and error handling preserve the stated invariant. No real financial transfer occurs in this lesson.
Commit the complete valid change or roll back the incomplete change. Two successful-looking partial steps are not enough if their combined invariant is broken.
Use this supplied case: A loses 3 and B gains 3 with no error. Atomic model moves 3 tokens from A to B. Classify the recorded outcome. The task is to connect the given information to the computing idea. Write a short evidence trail so another learner can reproduce your choice. The simplified case describes the stated model, not every real system.
- Represent the supplied worked case with a labeled drawing, table or short trace.
- Commit the complete valid change or roll back the incomplete change.
- Explain why the worked answer fits and why one alternative does not.
- Try the new check and practice cases; revisit the stated rule if your evidence does not match.
See the method in action
A fresh paper model has A=12 tokens and B=5. Moving 2 requires A=10 and B=7; the total stays 17. If the debit happens but credit fails, leaving A=10 and B=5 would lose 2 tokens from the combined record. Under the atomic policy, roll back to A=12 and B=5. Only a complete valid transfer commits. No actual money or learner record is changed in this model.
Check your reasoning
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Helpful tips
- Commit the complete valid change or roll back the incomplete change.
- Two successful-looking partial steps are not enough if their combined invariant is broken.
See the answer and explanation
Roll back incomplete transfer
Suppose account A has 10 tokens and B has 4. Moving 3 requires A=7 and B=7; the combined total remains 14. If the debit happens but the credit fails, keeping A=7 and B=4 loses 3 tokens and violates the complete-transfer requirement. Under the supplied atomic policy, roll back to A=10 and B=4. Only a complete valid operation commits. This paper model does not move actual money. The requested case is: A loses 3 butB update fails. Atomic model moves 3 tokens from A to B. Classify the recorded outcome. Its answer is Roll back incomplete transfer.
Take it further
New state: A=10 tokens and B=4. An atomic transfer of 2 requires A=8, B=6, combined total 14. If the credit fails, the stated rollback policy restores A=10 and B=4. Explain why a partial debit is unacceptable.
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More practice: try new cases
Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.
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Check the subject sources
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